With popularity of shooting functions on various kinds of portable devices, users now demand higher requirements for imaging quality. However, due to cost limitations on the portable devices, there is still a big gap in the quality between shooting lenses and sensors on the portable devices and those of professional cameras. In order to make up for the deficiencies of light transmission of the lenses and Signal to Noise Ratios (SNRs) of the sensors and to meet the requirement for lighting under low light conditions, various portable device manufacturers now adopt a greater maximum aperture, but the problem brought about by greater apertures is that depth of field of a focusing plane will be compressed correspondingly, that is, only a shallow scene of a shot picture is clearly focused, but the parts before and after that scene will be vague.